perf(streaming): batch main thread stream updates

[patrick-andstar]

Related to #4409

Why

The LangGraph SDK currently receives no throttle option from the main thread stream, which makes its effective setting false and notifies React for every stream update. The SDK's numeric throttle is a trailing debounce, so it can defer a continuously active stream instead of safely coalescing it.

What changed

The main thread stream now uses the SDK's throttle: true mode. It batches updates that arrive in the same macrotask without adding a fixed debounce interval. A focused regression test pins that hook configuration.

Surface area

• Frontend UI — page / component / setting / interaction under frontend/
• Backend API — endpoint / SSE event / request-response shape under backend/app
• Agents / LangGraph — agent node, graph wiring, langgraph.json, or prompt change
• Sandbox — docker/ or sandboxed execution
• Skills — change under skills/
• Dependencies — new/upgraded entry in backend/pyproject.toml or frontend/package.json (say what it buys us)
• Default behavior change — changes existing behavior without the user opting in (default model, default setting, data shape)
• Docs / tests / CI only — no runtime behavior change

Screenshots / Recording

Not applicable; this is internal stream scheduling with no new entry point.

Bug fix verification

• Test path that reproduces the missing configuration: frontend/tests/unit/core/threads/stream-throttle.test.ts
• Did it go red on main and green on this branch? No. The test is new and asserts the configuration contract that was absent from main.

Validation

• cd frontend && corepack pnpm test tests/unit/core/threads/stream-throttle.test.ts
• cd frontend && corepack pnpm format
• cd frontend && corepack pnpm check
• cd frontend && corepack pnpm test
• git diff --check

AI assistance

Tool(s) used: Codex

How you used it: Inspected the pinned LangGraph SDK implementation and types, implemented the narrow stream option change, added the regression test, and ran the listed validation and diff review.

• I've read and understand every line of this change and take responsibility for it — it's not unreviewed AI output.

[CLAassistant]

All committers have signed the CLA.